upgrade checkout action version

Author: OhKanghoon

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
             destination: platform=tvOS Simulator,name=Apple TV,OS=latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
 
     - name: Select Xcode 12.4
       run: sudo xcode-select -s /Applications/Xcode_12.4.app